1. Field of the Invention
The present invention pertains to a method of fabricating a microphone device, and more particularly, to a method capable of reducing cycle time of thermal oxide layer fabrication and reducing stress of structural layer.
2. Description of the Prior Art
Microphone devices formed by MEMS techniques have gradually replaced conventional microphone devices due to their small size and reliability. In various types of MEMS microphone devices, capacitive microphone device is the most popular one due to its high sensitivity, low self-noise, and low distortion.
With reference to FIG. 1 to FIG. 6, FIG. 1 to FIG. 6 are diagrams schematically illustrating a conventional method of fabricating a capacitive microphone device. For easy illustration, FIG. 1 to FIG. 6 merely illustrate one capacitive microphone device. As shown in FIG. 1, a substrate 10 is provided, and an oxide layer 12 is formed on the front surface of the substrate 10. The oxide layer 12 serves as a sacrificial layer to support a structural layer formed later, and the oxide layer 12 will be removed in successive process so as to form a suspended diaphragm. Normally, in order to let the diaphragm sharply response to the vibration due to a sound pressure, the thickness of the oxide layer 12 must reach several micrometers. However, it requires extremely long time to thermally form the oxide layer 12 of several micrometers thick. For instance, if the thickness of the oxide layer 12 reaches 4 micrometers, it requires about 50 hours to thermally form the oxide layer 12. As a result, the oxide layer 12 is normally formed by deposition. However, the deposited oxide layer 12 has mediocre physical characteristics. For example, the adhesion between the oxide layer 12 and the substrate 10 is poor, and therefore the yield is reduced.
As shown in FIG. 2, a structural layer 14, which serves as an upper electrode of the capacitive microphone device, is deposited on the oxide layer 12. Theoretically, the stress of the structural layer 14 increases as the structural layer 14 gets thicker, and therefore the thickness would affect the flatness of the structural layer 14. For fabricating a capacitive microphone device with a thicker structural layer 14, the structural layer 14 formed by conventional method is not qualified.
Referring to FIG. 3 and FIG. 4, FIG. 3 is a top view and FIG. 4 is a cross-sectional view along a tangent line AA′. As shown in FIG. 3 and FIG. 4, the structural layer 14 is partially removed by photolithography and etching techniques to define the location of a diaphragm. A shown in FIG. 5, the structural layer 14 is mounted on a support wafer 18 with a bonding layer 16. Subsequently, an anisotropic wet etching process, e.g. using potassium hydroxide (KOH) solution, to etch the substrate 10 made of silicon from the back surface so as to form a back chamber 20. The sidewall of the back chamber 20 is outwardly inclined as shown in FIG. 5. As shown in FIG. 6, a portion of the oxide layer 12 is etched off through the back chamber 20, so that the structural layer 14 is suspended.
The conventional method of fabricating capacitive microphone devices has the following disadvantages. First, to obtain a sufficient thickness, the oxide layer is formed by chemical vapor deposition. This leads to poor adhesion between the oxide layer and the substrate. In addition, the conventional method suffers from high stress and low flatness when a thicker structural layer is required. Furthermore, the sidewall of the back chamber is outwardly inclined, and this reduces the integration of the capacitive microphone devices.